1. Field of the Invention
The present invention relates to an integrated multimedia network system which integrates transport nodes with various communication media to construct a network between transport nodes.
2. Description of the Related Art
Conventionally, as a way to constitute a wide range integrated multimedia network system, various distributive queuing networks using a bus or ring have been proposed. Particularly, a distributive queuing network using fixed-length, short packets (hereinafter referred to as "cells") has a feature that can support real-time communication of voices, video images, etc. in addition to data, and is therefore suitable for multimedia environment. Of this type of network, the DQDB system (IEEE Draft 802.6. DADB MAC, Ver. C.O., Jan. 19, 1988) has been proposed as a system using a bus, and the Orwell ring system (R. R. Falcorner et.al., Orwell, British Telecom Technol J. Vol. 3, No. 4, October 1985) as a system using rings.
The DQDB system uses bidirectional double buses, and request an empty slot by a request bit to thereby keep fair accessing to the buses. In principle, this system has asymmetrical transmission path topology. When the load on the transmission path increases, therefore, fair accessing cannot be secured. If a hybrid type having a real-time mode which involves voices or video signals, that require real-time transmission, and a non-real-time mode such as data in which real-time transmission is unnecessary, is employed, it is difficult to provide highly flexible multimedia environment.
The Orwell ring system is basically constructed by fixed-length slotted ring transmission paths. At each transport node on the transmission paths is provided a so-called "d" counter, which decrements the count every time one cell is sent out. When d cells are sent out or no transmission cells exist, each transport node stops cell transmission. When every transport node stops sending cells at last, the "d" counters are reset, which permits all the transport nodes to be ready to transmit cells again.
This system has the advantage that it gives fair accessing to the ring of each transport node. Since the Orwell ring system needs to temporarily stop transmitting cells from every transport node, however, the throughput is reduced. In addition, this system also has such a shortcoming of the conventional ring system that as the load increases, the real-time transmission cannot be secured.
Recently has been proposed an ATM (A synchronous Transfer Mode) ring system (H. Ohnishi et.al., ATM Ring Protocol and Performance," IEEE ICC 89 Conference Record, 13.1.1., p. 394), an improved Orwell ring system. Although this system improves throughput compared with the ordinary Orwell ring system, it still has difficulties to deal with the flexibility of the system under a multimedia environment and assurance of different communication qualities for different terminals.